1. Field of the Invention
The present invention relates generally to dot-matrix printing, more particularly to scanning ink-jet color printing techniques, and more specifically to print-mode and masking techniques for improved photographic image quality printing.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy sic! Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
In dot matrix printing, the image to be printed is digitally composed by creating an array of picture elements ("pixels") arranged by rows and columns of dots, specified as a linear density, e.g., 600 dots per inch ("dpi"). In the following description, the word "ink" is used generically for any sort of colorant--e.g., ink, liquid toner, dye, dry toner, and the like as would be known in the art--employed by a hard copy apparatus to form the dot matrix.
FIG. 1 depicts an ink-jet hard copy apparatus, in this exemplary embodiment a computer peripheral printer, 101. A housing 103 encloses the electrical and mechanical operating mechanisms of the printer 101. Operation is administrated by an electronic controller (usually a microprocessor-controlled printed circuit board subsystem, not shown) connected by appropriate cabling to a computer (not shown). Cut-sheet print media 105, loaded by the end-user onto an input tray 107, is fed by a suitable paper-path transport mechanism (not shown) to an internal printing station where graphical images or alphanumeric text is created. Once a printed page is completed, the print medium is ejected onto an output tray 119. A carriage 109, mounted on a slider 111, scans the print medium. An encoder 113 is provided for keeping track of the position of the carriage 109 at any given time. A set 115 of ink-jet pens, or print cartridges, 117A-117D are releasable mounted in the carriage 109 for easy access. In the art, the term "print cartridge" usually designates a self-contained, disposable unit, including a substantial quantity of ink in an internal ink chamber (not shown) of the housing 103. In a "pen" type hard copy apparatus, separate, replaceable or refillable, ink reservoirs (not shown) are located within the housing 103 and appropriately coupled to the pen set 115 via ink conduits (not shown). The present invention is suited to either type ink-jet implementation.
As shown in FIG. 2, each pen 210 has a print head 214 which in turn has a orifice plate 216 and nozzle array 217 configuration; one of the main ink-jet design factors that controls droplet size, velocity and trajectory of the droplets. In a standard color printer, there is provided either one cartridge for each of yellow, magenta, cyan (subtractive primary colors) ink, or a single cartridge having three inks and grouped nozzle arrays (known as primitives) and one cartridge for black ink ("CMY" or "CMYK" hereinafter) are provided. A flex circuit 218 provides electrical interconnects 220 for interfacing the print head to a controller, e.g., a printed circuit board (not shown).
For photographic image quality printing ("PIQP" hereinafter), to be commercially acceptable--namely, to compete with true photochemistry prints--ink-jet printers must deliver artifact-free printing. Moreover, printing must be accomplished using a variety of print media (in order to facilitate the description of the invention, the word "paper" is used hereinafter as a generic terms for any print media such as plain paper, special paper, glossy paper, transparencies, envelopes, and the like), significantly more levels of color are required than occasioned by graphic image or alphanumeric text printing. Several approaches to obtain the necessary levels using standard ink-jet technology have been used, but all require that more drops of ink be placed on each imaging pixel than for non-PIQP image printing. This reduces throughput and uses more ink than is commercially desirable.
Systems have been designed and marketed in which a plurality of print cartridges provides inks having different chemistry in order to provide PIQP capability. That is, for example, a carriage may carry a print cartridge, or set of print cartridges, for cyan and magenta inks of a formula, and yellow ink and a separate print cartridge, or set of print cartridges, for cyan and magenta inks of a formula.sub.2 and black ink; six inks total. FIG. 3 shows a two-pen nozzle array arrangement (looking from a print media plane perspective). In one pen print head 301, the nozzle plate 216 has three, linear, double nozzle arrays, one set for yellow ink, one set for a first magenta ink formula, "M1," and one set for a second magenta ink formula, "M2." In the other pen print head 303, the nozzle plate has three similar arrays, one for black ink, "K," one for a first cyan ink formula, "C1," and one for a second cyan ink formula, "C2." Such a color printer system is available from Hewlett-Packard.TM., designated as a PhotoSmart.TM. color printer model.
In ink-jet technology, "print-mode" techniques basically refer to the optimization of image quality by the use of specific multiple or partial-inking patterns where dots are printed in each pass of a pen with only a fraction of the total ink required in each section of an image. In essence, print modes distribute rather than accumulate print-mechanism errors that are impossible or expensive to reduce. The result is to minimize the conspicuousness of errors at minimal commercial cost.
For example, one early print mode was dubbed double-drop always. Each pixel was dotted by one nozzle of a primitive in a first pass and by another nozzle in a second pass, ensuring at least one drop of ink on the pixel if a defective drop generator existed in a pen or set of primitives therein.
Another particular print-mode is to divide up a desired amount of ink into more than one pen pass in a checkerboard pattern. Every other pixel location, or groups of pixels referred to as superpixels, is printed on one pass and the blank pixel locations filled in on the next pass or passes (referred to generally as multi-pass scanning or printing). Checkerboarding helps alleviate the problems of bleed (one color running into an adjacent color; particularly noticeable at color boundaries that should be sharp), blocking (transfer of wet ink in one printed image onto the back of an adjacent sheet in an output stack with consequent sticking of the sheets and smearing of lower sheets), and cockle (puckering of the printed paper due to excess ink carrier absorption). However, checkerboarding tends to exhibit moire pattern errors (print artifacts having regular frequencies, harmonics, etc., generated as a result of regular interacting subsystem interfaces, e.g., a periodic pen carriage drive belt vibration).
In a slightly more advanced technique, checkerboarding is combined with sub-swath width, or sub-pixel width, paper advances between an initial swath scan and a fill swath scan or scans. This technique helps alleviate the problem of banding (noticeable boundary stripes, rather than smooth transitions, between swaths). Interference effects such as moire patterns are still evident with such techniques.
Even more complicated print-mode techniques appear in the literature and the prior art.
The pattern used in printing from each primitive section or pen nozzle array set is known as the "print-mode mask," or "print mask(s)" for short. That is, a print mask is a component of a print mode. Masking relates to assigning particular ink-to-pixel applications to particular printing device operations. In effect, a print mask selects operating parameters such that pixel dotting is optimized. The print mask also defines the number of passes required to reach full dot density, the maximum number of drops per pixel. A number of print masking techniques have been developed. A general summary is provided in U.S. Pat. No. 5,555,006 (Cleveland et al., assigned to the common assignee of the present invention and incorporated herein by reference) at column 4, line 19, through column 7, line 65.
Print masks are generally divided into two types: spatial and temporal. Spatial masks contain the drop deposition patterns for all passes of a multi-pass scanning print head pen for all passes of a swath for each pen, or primitive, full nozzle array. Temporal masks have a separate mask for each drop deposition pattern with a full nozzle array for each separate pass over a swath.
Print modes and print masks are used in conjunction with color maps (for example, red, green, blue (RGB) color triplet data to CMY color triplet data conversion). Such color maps are known in the art. Thus, a print mask identifies and controls when the required ink drops are fired based upon the data processing that has occurred in the controller using the color maps.
There is a need for improvement of print masking techniques in order to allow the placement of multiple drops of ink from more than the traditional CMYK color printer designs in order to achieve PIPQ without requiring excessive use of ink.